Vitamin B12, in the form of its coezyme, is a cofactor in a series of eleven enzyme-catalyzed transformations. Of these, three enzymes: Beta-methylasparate mutase, methylmalonyl-SCoA mutase and methylitaconate mutase are of special interest because they catalyze carbon-skeleton rearrangements for which there has been no analogue in organic chemistry. In this proposal, a new set of chemical, nonenzymic model reactions will be explored with the aim of unravelling the mechanism of action of vitamin B12. It is proposed to pursue the non-enzymic model for the Bata-methylaspartate rearrangement to be studied itself and as a reactive intermediate in the enzymic reaction. New cyclopropyl models for the methylitaconate rearrangement will be constructed. The methylitaconate model will be examined using carbon-13 labelling. The hydrogen abstraction reaction characteristic of the B12 series will be probed using models. The possible intermediacy of free radical intermediates will be explored using models and the tools of electron spin resonance and chemically induced dynamic polarization.